In many situations, it is desirable to move a controlled member to a predetermined position in response to a system failure. Since this predetermined position is often dictated by safety considerations, actuators which have been developed to achieve this objective are commonly termed "fail-safe" actuators.
Fail-safe actuators are presently employed in a wide variety of situations. For example, such actuators may be used to move a valve or other controlled member to either an open or closed position in response to an electrical or hydraulic failure. Similarly, as a further example, a fail-safe actuator may be used to move a controlled member, such as a valve in a process line for combustible media, to a predetermined position in response to a fire.
Typically, fail-safe actuators rely upon stored energy to return a controlled member to a fail-safe position in response to system failure. The most prevalent energy storage means in contemporary fail-safe actuators is a return spring wherein the spring is arranged to urge the controlled member to the desired fail-safe position. During normal operating conditions, an externally generated force is applied against the return spring to overcome the spring bias. The externally generated force is terminated in response to a sensed system failure, and the stored spring force is then used to return the actuator to the predetermined fail-safe position. Spring return fail-safe actuators are highly advantageous in that the stored energy used to move the controlled member to its fail-safe position is not normally immediately effected by the source of system failure.
An exemplary prior art spring return fail-safe actuator is disclosed in U.S. Pat. No. 3,542,331 to Canalizo. In this disclosed arrangement, a pressurized fluid is applied against a piston to move the piston against the biasing force of a compression spring. The rectilinear movement of the piston is converted into rotary movement to cause a controlled valve member to be rotated about an arc of 90 degrees and to move the valve from a closed position to an open position. When the pressurized fluid pressure against the piston is relieved in response to predetermined conditions or signals, energy stored in the compressed spring is used to move the controlled valve member back to its closed position.
Many applications require the controlled member to be repeatedly moved or cycled during normal operation. Movement of the controlled member, such as cycling the valve of the above described Canalizo arrangement between open and closed positions, has also required movement of the return spring in most prior art arrangements. In addition to introducing wear and increasing the possibility of fatigue failure in the return spring, cycling of the spring also increases the demands placed upon the actuator, since the biasing force of the return spring must be overcome in order to move the controlled member. A further example of a spring return actuator wherein a return spring is cycled with the controlled member during normal operation is disclosed in U.S. Pat. No. 3,051,143 to Nee.
A fail-safe actuator which does not require cycling of the return spring with the controlled member is disclosed in U.S. Pat. No. 4,295,630 to Card et al. In this arrangement, a flexible cable is used to couple a spring biased piston to a rotatable torque transmitting shaft. An end clamp is used to retain one end of the cable in a recessed, radially outwardly open track in an arcuate sector of a yoke, the yoke being attached to the rotatable shaft. The opposite end of the cable is secured to the piston. The piston is spring biased to an extended position by a compression spring. In operation, a pressurized fluid is introduced into the Card et al actuator to move the piston to a retracted position against the bias of the compression spring. When the piston is in this retracted position, the flexible cable is in a slack position which permits cycling of the rotatable shaft (and a controlled member) without the necessity of cycling of the piston and compression spring. When the pressurized fluid is discharged, the compression spring returns the piston to the extended position, bringing the cable into a tautened condition and moving the rotatable shaft and a controlled member to a fail-safe position.
The advantages offered by an arrangement which permits movement of the controlled member during normal operation without the necessity of moving the return spring are significant. However, the arrangement utilized in the above identified Card et al disclosure has several shortcomings, particularly in high torque applications. As the size of a cable increases to meet increased torque requirements of a valve, for example, the flexibility of the cable diminishes. Not only does the cable become difficult to bend about an arcuate yoke, the cable also tends to transmit force to the piston instead of flexing as the controlled member is moved during normal operation. Additionally, it is difficult to reliably secure an end stop onto a cable, or otherwise reliably secure the cable to the yoke or the piston, to withstand the tension generated in high torque applications.
Furthermore, actuators of the type discussed above typically have used springs having linear force-displacement characteristics. Accordingly, the return force exerted by the spring varies linearly throughout the return stroke with the minimum spring force corresponding to the return or fail-safe position of the controlled member. In many situations, however, the force requirements of the controlled member varies non-linearly and does not match the spring force-displacement characteristics. For example, a plug valve is generally rotated approximately 90 degrees between fully open and fully closed positions with maximum torque requirements occuring at both ends of the rotational stroke. If a linear spring is used to return a plug valve from a fully open to a fully closed position (or vice versa), the maximum spring force would match the maximum torque requirements only at the beginning of the return stroke (as the valve begins to move from the fully open position to a closed position). At the opposite end of the return stroke (as the valve is moved through the final phases of its rotation to the fully closed position), the return spring force is at a minimum while the torque requirements of the valve once again reach a relative maximum level. Moreover, many applications encounter high dynamic forces wherein the maximum torque (or linear force) requirements are encountered intermediate the end positions of the return stroke. As a consequence of the mismatching of the spring force output and the valve torque requirements, designers have been relegated to oversizing the return spring to insure adequate spring force throughout the entire range of valve or other controlled member movement. The need to oversize the return spring is particularly disadvantageous when the spring must be moved along with the controlled member during normal operation.